In modern astronomy, an important activity is the search for and study of exoplanets. An exoplanet is a planet in orbit around a star other than the sun. A difficulty encountered in the exoplanet study effort is the fact that the star is significantly brighter and larger in size than any exoplanet in orbit around the star. Distortions of the star image further complicate exoplanet study. For example, a point spread function (PSF) of an optical system is the irradiance distribution on a recorded image that results from a single point source in object space. A star in space is so far from earth, that it appears as a point to an earthbound telescope, or even to a telescope in orbit around the earth. However, due to inherent aberrations in the optics of a telescope, the image of the point source comprising the star, is spread over a finite area of a sensor receiving and recording the image from the optics of the telescope. Thus, a bright star whose image is distorted by PSF, presents significant difficulties to an accurate and correct study of an exoplanet orbiting the star.
One solution that is of particular benefit in exoplanet exploration and study is the coronagraph. A coronagraph is a telescopic attachment designed to block out the direct light of a star. An occulting mechanism is placed at the focal and other planes of the telescope to prevent light from the star from impinging upon downstream optical sensor elements of the telescope. Thus, the light intensity of the star does not completely obscure an exoplanet, thereby facilitating study of the exoplanet.
However, a further difficulty arises from the use of a coronagraph when study of an exoplanet is desired in respect of astrometry and photometry. Astrometry concerns the analysis of stars and other celestial bodies in terms of their distances and movements. Accurate relative astrometry between a star and an exoplanet in orbit about the star or the extended disk structure around the star, is important to an understanding of physical association between these celestial objects. Confirmation, by means of astrometric techniques, of such factors as common proper motion and common parallax, provides a basis for determination of orbital parameters and observation of disk structure quantitatively. Photometry is a technique of astronomy concerned with the measurement of flux, or intensity of the electromagnetic radiation of an astronomical object. Relative photometry between a star and objects in orbit around the star is useful in studies of disk and/or exoplanet physics and chemistry.
However, a perfect coronagraph operates to completely obscure the image of the PSF of a star. Thus, measurements of the intensity and location of a star, for use in astrometry and photometry, are typically either not possible or difficult, while using a coronagraphic image. Techniques have been proposed, for example, extreme adaptive optics (ExAO), for analysis and study of phenomena such as exoplanets, however, the proposed techniques are complex and difficult to implement.